The present invention generally relates to a voltage/frequency converter (hereinafter referred to as V/F converter) for converting the instantaneous value of an input potential into frequency information.
In a frequency modulation system, the frequency of a carrier wave is modulated by the instantaneous value of an input potential and the modulated carrier wave is transmitted to a remote place. In such a modulation system, the carrier frequency is increased with increase of the present value of the input potential and reduced with reduction thereof. Although various types of frequency modulation systems have been known, either of these systems basically have an input/output characteristic as shown in FIG. 4.
As will be seen from FIG. 4, when the input potential (input level e10) is at zero, the output frequency (fc) is fo (Hz). Output frequencies fH (Hz) and fL (Hz) are obtained at positive and negative peaks emax and -emax of the input potential, respectively. Thus, the output frequency (fc) can be deviated by .+-..DELTA.f (Hz) from the center frequency of fo (HZ) (.DELTA.f=fH-fo=fo-fL).
A V/F converter circuit used in such a frequency modulation system is conventionally made of a linear circuit (IC), and a drift of the operating point of the circuit may happen. The drift is liable to be increased not only according to ambient conditions, such as temperature, but also in long use. Such a drift causes a frequency shift in the V/F converter output. Namely, even if the input potential of the V/F converter is at zero, a frequency shift (fd) corresponding to the magnitude of a drift appears in the converter output frequency. From this, when the input is varied from -emax to emax in this situation, the output frequency is varied from fL+fd (Hz) to fH+fd (Hz).
When such a drift appears, an output frequency, exactly corresponding to the input potential, can no longer be obtained, resulting in posing problems in data transmission. Suppose a case where data in a form of a series of signals having a predetermined frequency is transmitted with a certain drift, for example. Then, the data is transmitted with an erroneously shifted transmission frequency, so that the transmitted data is inaccurately confirmed at the receiver side. Similar problems may arise when a drift is involved in the input potential per se.
The V/F converter output has a form of a signal series with instantaneously varied frequencies. If it is intended to effect a drift compensation through data processing of the signal series, an enormous drift compensation circuit is required. Consequently, it has been desired to develop a compensation circuit, which can achieve the drift compensation without complicating the circuit configuration.